Lee Byeong Chan (Byeong-Cheol Kim) is a brilliant scientist who also works as a substitute teacher at Hyosan High School, where his son is often bullied. When his son attempts suicide, he starts to experiment on him in order to discover a method for him to fight back, which leads to the development of the Jonas virus.
A student is bitten by a rat that was part of Byeong Chan's research one day at school. He restrains her at first, but she breaks free and returns to her classroom before being transported to the infirmary. They call an ambulance, but she bites the school doctor and transmits the infection before being transferred.
Slowly but steadily, the school's kids and teachers become infected and go insane, while patient zero, who was transferred to the hospital, begins to transmit the virus around the town at an alarming rate.
A group of students seeks refuge in the science lab, trying to make sense of what is going on. Byeong Chan has been arrested and is being interrogated by detective Song Jae Ik (Kyoo-Hyung Lee). Byeong Chan warns him that if the infection spreads, there is no way to stop it.
The children in the science lab decide that they can't remain any longer and climb down from the outside using a water hose. They descend two stories to the transmitting room, where they meet Mrs. Park, their teacher (Lee Sang-hee). They have to fend off a zombie that is following them, and Gyeong-Su (Ham Sung-min) is suspected of being bitten in the process.
Despite pleading his innocence, he accepts to isolate himself in order to prove it. Na-Yeon (Lee Yoo-mi), the group's most suspicious member, is instructed to apologize, but she instead infects him by cleaning his open wound with zombie blood. The team has lost three members as a result of this contact.
So-ju (Jeon Bae-soo), a firefighter, is dispatched to save an assemblywoman, but he's more concerned with saving his daughter, On-jo (Park Ji-hoo), from the school. After being persuaded to delay his choice, he is given the order to wait until the next day for the rescue chopper. They make plans to spend the night together.
Cheong-san (Yoon Chan-young) and Soo-hyuk (Park Solomon), two of the students, decide to descend to the teacher's lounge and obtain a phone. They are able to obtain the phone, but Cheong-san is prevented and must flee. The infected have gained access to the police station, and Byeong Chan informs the investigator that the only potential solution is on his laptop at school.
He then puts his own life on the line to save the investigator. Jae Ik needs to rush to the emergency center and inform them about Byeong Chan and his laptop since martial law has been imposed in the region.
Cheong-san witnesses Gwi-Nam (Yoo In-soo) killing the principal and then attempting to flee while evading the zombies. Cheong-san watches as Gwi-Nam is overrun by zombies after stabbing him in the eye. He makes it to the music room and sets up a blockade there.
With the aid of a drone, the others in the broadcasting room figure out where Cheong-san is and devise a strategy to approach him via the school speakers. They entice the zombies to one side of campus and charge up the opposite side. Gwi-Nam, who bites Nam-Ra before being thrown out the window, holds up Soo-hyuk and Nam-Ra (Cho Yi-Hyun).
Others are concerned about Nam-Ra, but Soo-hyuk swears she will not transform because Gwi-Nam was not acting in a zombie-like manner. So-Ju escapes the quarantine camp in order to get to his daughter's school and save her.
The kids decide to climb to the top with the expectation of being rescued, so they devise a plan to construct a barrier in the center and use music to draw the zombies in. When all of the zombies have entered, they flee out the opposite side of the room and climb to the roof. They arrive too late, as the rescue chopper that had flown above them departs before noticing them.
On his way to the quarantine camp, Jae Ik stumbles across an abandoned newborn, a worried little girl, and a stranded social media personality. They take over a bus and go throughout the city. They come upon Eun-Ji (Oh Hye-soo), an infected student who, unlike Gwi-Nam and Nam-Ra, does not display any symptoms.
Soon later, they are apprehended by a group of troops and hauled to the camp. When Jae Ik informs them about the laptop, they sent a team to the school to collect it. They intend to save the students on the rooftop once they have it secured, but Eun-attack Ji's on someone at the camp casts doubt in the soldiers' minds, and they are told to depart without them.
A thunderstorm strikes later that night, allowing the kids to devise a fresh strategy for escaping the school grounds. They use the turmoil of the storm to go to the auditorium, where they meet up with another set of survivors but are trapped in a chamber once again.
The very next morning, they find out how to use the steel trolleys to construct a barrier around them so that they can get to the rear door and escape. They reach to the other side, where So-Ju appears and saves them from a dangerous scenario, but not without further losses.
So-Ju leads them to the mountains and offers himself as a sacrifice to assist them in their escape. They make it to a nearby work site, but the zombies trap them down once more. The military chooses to concentrate all of the zombies into four large zones and blast them. They utilize drones that produce sound at a specific frequency to draw everyone in.